1. Field of the Invention
This invention relates to testing flexible substrates with circuit elements, and, more particularly, to apparatus tensioning such a substrate within a frame and for applying circuit tests by means of an array of probes simultaneously contacting various points within the circuit from each side of the substrate.
2. Description of the Background Art
Certain problems associated with the testing of flexible printed circuit substrates are addressed in U.S. Pat. No. 5,467,020, which describes a mechanism in which a flexible circuit substrate is fed between an upper plate and a lower plate for circuit testing applied by movable, or flying probes. The upper plate includes a number of upper apertures extending across the flexible substrate and a number of upper segments, also extending across the flexible substrate, between adjacent apertures. The lower plate includes lower segments extending under the upper apertures and lower apertures extending under the upper segments. Two upper test probes are moved above the flexible substrate, while two lower test probes are moved under the flexible substrate. Tests are applied to both sides of the flexible substrate as the probes are brought into contact with test points in the areas accessible through the upper and lower apertures, with segments extending along the apertures on the opposite sides of the flexible substrate providing a backing surface for probe contact.
Thus, while U.S. Pat. No. 5,567,020 describes a mechanism providing a backing surface for probing from each side with flying probes, a mechanism facilitating the use of large numbers of probes in a stationary configuration, which is often capable of providing test results for a dense, complex circuit much more quickly, is not described. Furthermore, while this patent describes a system testing circuits extending along a long substrate, an efficient method for handling large numbers of discrete substrates is not described.
A number of prior art publications and patents describe frames for tensioning flexible circuit substrates and the like. For example, a work board for holding a flexible circuit flat to provide compatibility with automated, robotic assembly and handling during the manufacturing processes of building a populated working flexible circuit is described in Article 37535, RESEARCH DISCLOSURE, March, 1987, number 275, Kenneth Mason Publications Ltd., England. This work board includes a frame having two holes, spaced at mid-points along opposite sides, receiving pins extending through holes in the flexible circuit substrate to locate the substrate. At each corner of the work board, a spring-loaded, low profile slider hooks into an additional hole in the flexible circuit substrate, which is tensioned by pulling all four corners as it is held in place at mid-points along opposite sides.
However, this work board includes a number of mechanical clamping devices, indicating that the tensioning concepts described in this article would be particularly difficult to apply within a testing device requiring a rapid, automated attachment of the substrate to a board used to transport the substrate through testing processes, and rapid, automated detachment of the substrate from the board. Furthermore, six special holes must be placed in the circuit being manufactured for engagement with these clamping devices.
U.S. Pat. Nos. 4,667,944, 4,711,044, and 4,778,326 describe apparatus for handling semiconductor chips and like objects. In U.S. Pat. Nos. 4,667,944 and 4,711,044, the object to be handled is placed on a flat flexible film that is supported on a flat, porous texturized porous texturized fabric sheet, such as a woven or knit fabric having spaced fiber crossovers. In U.S. Pat. No. 4,778,326, the object to be handled is attached to the face of a base member, with the face being formed with recesses covered by the flat flexible film. In the apparatus of all three patents, objects are supported on the flat flexible film in intimate surface contact therewith for securely holding the same in position by interfacial forces therebetween. Adhesive may be included for increasing the interfacial force. To facilitate removal of objects from the film, the fabric of U.S. Pat. Nos. 4,667,944 and 4,711,044 is connected to a vacuum source for drawing portions of the flexible film over and between crossovers in the fabric. Similarly, the recesses of the face of the base member described in U.S. Pat. No. 4,778,326 are connected to a vacuum source, drawing portions of the flexible film into the recesses and providing the film with a textured, or undulating surface. In any case, the application of a vacuum causes portions of the film to be withdrawn from the object being held, reducing the area of contact between the flexible film and the object being held, and hence the interfacial forces therebetween. In this way the removal of the object using a conventional technique is enabled.
While the methods of U.S. Pat. Nos. 4,667,944, 4,711,044, and 4,778,326 are particularly effective in facilitating the handling of small, flat devices, such as semiconductor chips, what is needed is an effective method for handling relatively large flexible sheets, which may include, for example, a number of identical circuits to be cut apart when processing is finished. Since the methods of these three patents are designed for small chips, a need to hold flexible sheets in a taut, tensioned condition is not addressed. An effective, automated system for attaching flexible sheets to frames holding such sheets in a tensioned condition, for transporting such frames with sheets attached therethrough into and out of an inspection process, and for subsequently removing the sheets from the frames is also needed.
Apparatus for applying tests with two probes on each side of a vertically oriented rigid circuit board is described in the IBM Technical Disclosure Bulletin, Vol. 14, No. 12, May 1972, pp. 3623-3624. However, this apparatus does not include means for handling flexible circuit substrates, or for moving substrates through the test process in a fast, efficient manner.
U.S. Pat. No. 4,998,885 describes an interposer for electrically connecting two substrates with high density interconnections. The interposer comprises an elastomeric material surrounding fine metal wires which extend through the elastomeric material. The elastomeric material provides mechanical support and electrical isolation for the wires. What is needed is a mechanism using the device described in this patent to apply testing signals, and to sense resulting signals, in a circuit under test, with a number of copies of the circuit being presented in a rapid, automated, and efficient manner to the interposer.
U.S. Pat. No. 5,434,504 describes position sensors for detecting the position of a linear motor relative to a platen. In particular, these sensors are described in an arrangement including a Sawyer motor forcer. What is needed is a mechanism using a positioning arrangement of this kind to position large number of circuit test probes.
The above-described examples of prior-art devices do not describe means for simultaneously applying a large number of probes to opposing sides of a flexible circuit substrates, for moving a substrate so that various portions thereof are presented to fixed probe configurations, or for marking individual portions of a substrate which is divisible into a number of discrete segments so that those segments failing circuit tests applied thereto are not inadvertently used following the test process. Furthermore, none of these examples of prior-art devices include a mechanism for aligning the test probes in response to visual indications on the circuit board.